1. Field of the Invention
This invention relates to a tube bending machine, and more particularly, to a tube bending machine performing a bending work by applying pushing or tensile force to a tube to be bent.
2. Description of the Prior Art
The bending of a metallic tube holds an important position in the industry such as the vehicle or automobile manufacturing industry. Namely, tubes installed in a vehicle are required to be bent lengthwise accurately at their predetermined positions in order to route them while avoiding other parts or to connect them to other parts.
In the prior art, wherein a tube is bent for these purposes, the tube is bent as established beforehand with a gripping means which grips, moves, and sets the tube at a predetermined position and with die means which bend the tube at the predetermined position by a prescribed amount, and after the bending work is completed, the dimensions of the bent tube are measured with a three-dimensional measuring apparatus, wherein if correction is required, adjustments are made in the subsequent bending process with regard to the amount of movement and setting df the gripping means as well as the bending amount of the die means.
In the Japanese Laid-Open Patent Application No. 63-290624, a tube bending method and a tube bending machine have been proposed wherein a position sensing means detects a prescribed position between one end of the tube and the bending position after the tube is bent as established beforehand with a gripping means which grips, moves, and sets the tube at the prescribed position and with die means which perform the bending work at the bending position by a prescribed amount of bending.
In the method proposed by the said Application, the position sensing means detects a position after the tube is bent, and a control means modifies the bending amount of the die means so that the detected position comes to be the preset value and the predetermined bending work can be realized.
In the aforesaid conventional method, since the bending amount is adjusted by measuring the bent tube dimensions with the three-dimensional measuring apparatus after a first tube is bent, this tube cannot be used as a product in many cases. In the case of comparatively short tubes, it is possible to use them as a product, but in the case of long tubes, the bending amount is required to be readjusted each time the bending work is performed since the work is affected by differences in hardness, elastic limit, and yield force due to variance in the material, wall thickness, and heat treatment conditions of the tubes. Furthermore, in the case of small-diameter tubes, correct dimensional measurements may not be possible since deflection occurs with such a tube when measured with the three-dimensional measuring apparatus. On the other hand, in the method proposed by the Japanese Laid-Open Patent Application No. 63-290624, it becomes practically difficult to apply the method to long tubes because the position sensing means must be made very large in size, while with small-diameter tubes, deflection increases at areas closer to the tube end after the bending work is performed, so that high-accuracy position sensing is no longer possible.